This invention relates generally to the field of superconductivity, and more specifically relates to a novel multifilament superconductor and its method of manufacture.
Composite superconducting wire characterized by multiple longitudinally extending strands of high field superconducting compound in a surrounding matrix of a good thermal conductor such as aluminum or copper has found application in a number of important fields, e.g. in superconducting magnets capable of producing extremely high magnetic fields. Similarly, such composite wire has been of great interest in connection with many electrical applications, notably including for use in power generation and transmission.
In the foregoing connection, it may be observed that while the phenomenon of superconductivity has been known for over half a century, the applications of this striking discovery have in totality been quite limited. A principal if not the major reason underlying such limited application has been that until very recently known superconductors have exhibited very low transition temperatures. Indeed, until such recent times, virtually all practical superconducting materials exhibited critical temperatures (Tc) requiring the use of liquid helium as a refrigerant.
Within recent years, however, a number of Type II superconductors have been reported, with relatively high transition temperatures. Thus, for example, in approximately 1953 the compound V.sub.3 Si was reported and found to become superconducting at 17.1.degree. K. Not long thereafter, the compound Nb.sub.3 Sn was developed. Like V.sub.3 Si, the latter is a type A-15 compound, and exhibits a Tc of 18.degree. K. More recently, a ternary A-15 compound, Nb.sub.3 Al Ge with a Tc of 20.05.degree. K has been found -- with the critical temperature thereof reportedly being raisable somewhat further by specific fabrication methods. A further compound, Nb.sub.3 Ga, another A-15 type, was reported in 1971, and found to be superconducting at approximately 20.3.degree. K. Yet more recently, there was announced the discovery that the compound Nb.sub.3 Ge (also an A-15 Type) could maintain it superconductive properties at temperatures of 23.2.degree. K at which temperature liquid hydrogen can be used as the refrigerant.
The A-15 structure, as is well-known to investigators in the present art, represents a large fraction of known superconducting compounds. The A-15 unit cell is thus based on the stoichiometric A.sub.3 B ratio, in which all (1/2, 1/4, 0) sites are occupied only by A atoms, and all (0, 0, 0) and (1/2, 1/2, 1/2) sites are occupied solely by B atoms. It of course will be understood that this idealized stoichiometric structure is in general not completely achieved in any real fabrication technique.
The Nb.sub.3 Ge compound cited, because of its unusually high Tc characteristic, has been the subject of considerable investigation. Reference may be had in this connection to the technical papers "Superconducting With Onset Above 23.degree. K in Ng-Ge Sputtered Films" by L. R. Testardi et al, Solid State Communications 15(1): 1-4 (July 1, 1974); and "Superconductivity in Nb-Ge Films Above 22.degree. K" by J. E. Gavaler, Applied Phys. Letters 23(8):480(1973). The two cited papers are among several that report the use of D.C. sputtering techniques in a high purity environment for the purposes of producing what appears to be a nearly perfect stoichiometric Nb.sub.3 Ge compound. Investigators in the art have tentatively concluded that it is the fact that the films sputtered by the reported technique are in a more crystallographically ordered state than has been possible with prior techniques for bulk formation of Nb.sub.3 Ge films, and reference should be had to the cited papers for details of the sputtering process. These papers are thus incorporated by reference into the present specification.
While the Nb.sub.3 Ge sputtered films are therefore of enormous interest to practitioners in the present art, it has been found in practice that such films are extremely brittle -- which indeed is also the case with other superconducting materials possessing the A-15 structure. While this property can be tolerated in other A-15 compounds such as Nb.sub.3 Sn because the Nb and Sn can be reacted in situ after the multifilament structure has been worked to its final form, this is impossible with Nb.sub.3 Ge which must be sputtered to achieve the ordered structure necessary for the desired high Tc.
Other approaches have been used to produce a plurality of superconductive filaments of the A-15 structure on a substrate. One such approach is shown in U.S.S.R. Patent No. 410,505 which was published May 12, 1974 and is incorporated herein by reference. The method described in that patent produces a flat superconductor cable with a plurality of parallel spaced superconductor ribbons which are vacuum vaporized and condensed on a grooved flat substrate. During film depositing, the substrate is disposed at an angle to the vapor source so that a portion of each groove remains deposit-free due to geometric shadowing thereby providing a plurality of spaced superconductor ribbons on the substrate.
This approach is not suitable for making superconductor wire especially where the superconductor filaments spiral around the conductor because the grooves cannot be maintained at a constant angular disposition to the source of superconductor material. The approach described above, when applied to wires, causes bridges of superconductor material to be formed between adjacent superconductor portions on the exterior of the conductor. A further disadvantage of this approach is that vacuum deposition is not suitable for depositing all types of films because the desired chemical composition of the film cannot always be maintained.